A selective catalytic reduction (SCR) system incorporating an SCR device that has been developed is an exhaust gas purification system that purifies exhaust gas of a diesel engine of NOX.
The SCR system is to purify exhaust gas of NOX by supplying urea solution stored in a urea tank to an upstream part of the exhaust gas and reducing the NOX on an SCR catalyst with ammonia derived from the urea solution by the heat of the exhaust gas (see Patent Literature 1, for example).
A process of deriving ammonia from urea solution will be described below with reference to FIG. 6.
As shown in FIG. 6, as the temperature of the urea solution rises beyond the boiling point, water starts evaporating, and the urea solution transforms into melt and then to gas. The resulting gas crystallizes into solid as the temperature further rises. The resulting solid then sublimates into ammonia. Part of the melt and gas are hydrolyzed with steam to form ammonia and carbonic acid gas.
The urea solution is injected through a dosing valve (a urea injection device or a dosing module) provided at the upstream side of the SCR device.
As shown in FIG. 7, a dosing valve 104 includes a cylinder 127 having an injection orifice 128 and filled with urea solution at high pressure, a tubular plunger 130 having a slit 133, and a valving element 129 to close the injection orifice 128 that is attached to the plunger 130. A coil 131 is energized to pull up the plunger 130 to separate the valving element 129 from the injection orifice 128, thereby injecting the urea solution. Furthermore, a spring 132 is provided in the dosing valve 104 to bias the valving element 129 to be normally closed via the plunger 130. Therefore, when energization of the coil 131 is stopped, the plunger 130 is pushed down by the spring to make the valving element 129 close the injection orifice 128, thereby stopping injection of the urea solution.
The urea solution is supplied from the urea tank to the dosing valve 104 by a supply module (“SM”), which includes an SM pump, a urea solution pressure sensor and the like. More specifically, the urea tank and the supply module are connected to each other by a liquid feed line, and the urea solution sucked up from the urea tank through the liquid feed line is supplied to the dosing valve 104 through a pressure-feed line that connects the supply module and the dosing valve 104 to each other. In this process, the pressure of the urea solution in the dosing valve 104 (the measurement value of the urea solution pressure sensor of the supply module) is feedback-controlled to be constant, and injection of the urea solution starts when the pressure becomes constant.